1. Field of the Invention
This invention relates to a micrometer gauge or caliper for measuring dimensions of a workpiece to be measured including a length, a thickness and the like through a displacement of a spindle.
2. Description of the Prior Art
Heretofore, there have been developed micrometers of various types. One of the most commonly used ones is a so-called thread micrometer wherein an inner sleeve affixed to a main frame is internally threaded with precision, and external threads formed on a spindle also with precision are threadably coupled to the internal threads thus formed, whereby the spindle is rotated by means of a thimble integrally secured to the spindle to measure a workpiece to be measured. The thread micrometer of the type described can offer the advantages that the micrometer is excellent in dust exclusion because an internal mechanism including the threads is of substantially sealed construction, and, even if an operator releases his hand from the thimble, the spindle is prevented from freely rotating due to a self-locking action of the threads so that the workpiece is secured in its clamped state.
However, on the other hand, the pitch of the threads is generally as minute as about 0.5 mm, and the values of bite between the threads, i.e., the coupled positions of the the threads are varied due to the amount of the force for operating the thimble when the zero point is set or the workpiece is clamped, which results in an unstable measuring accuracy, and consequently, a high skill level is required for the measuring operation.
Since the thread pitch is so minute as described above, it is impossible to move the spindle at high speed, and particularly the measuring efficiency is low in repeated operations. In order to obtain a high speed performance by roughening the pitches of the threads formed on the inner sleeve and the spindle, and moreover, maintain the same level of precision as before, it is necessary that the graduations formed on the peripheral surface of the thimble be made finer than what they were in proportion to the roughened pitches. For example, to read 0.1 mm by use of a commonly used micrometer in which the thread pitch is 0.5 mm, the graduations formed on the peripheral surface of the thimble may be ones obtained by dividing the peripheral surface into fifty equal parts. If the thread pitch is increased by ten times, then the same level of precision cannot be obtained unless the peripheral surface of the thimble is divided into five hundred equal parts. As a result, the high speed performance could not be attained in fact.
No operation at high speed can be attained as described above, and moreover, the spindle is allowed to rotate during measuring operation because the spindle is directly threaded, which results in wrinkles and the like formed on the workpiece which is made of a flexible material such as a soft plastic plate, during the measuring operation. Consequently, the thread micrometer of the type described is not suitable for measuring the workpiece made of such materials as described above, and the thimble also rotates and moves in the axial direction of the spindle during measuring operation. The construction described above is unfit for a micrometer construction to be handled by a single hand. Additionally, there is presented such a disadvantage that precision finishing required for the threads and graduations leads to high costs.
Furthermore, there is encountered with a complicated measuring operation during which the graduation and the vernier formed on an outer sleeve coupled to the inner sleeve and on the thimble, respectively, should be read.
Now, there has been known a so-called linearly movable type micrometer capable of moving the spindle in the axial direction thereof at high speed without allowing the spindle to rotate. However, the conventional linearly movable type micrometer has been constructed such that a control knob formed at a predetermined position relative to the spindle is caused to project to the outside of the main frame, this control knob is operated by a thumb or the like to move the spindle in the axial direction, while a movement value of the spindle is detected and indicated by an indicator or the like driven by a rack and a pinion. In consequence, it is necessary to provide a lock device for the spindle, a snap handle for repeated measuring and a measuring pressure applying device in particular, and moreover, the measured value fluctuates due to the force applied to the control knob, thus preventing reliable measured values.